The invention concerns a device for screening fiber suspensions with a rotationally symmetric screen basket and a coaxial rotor arranged radially inside said basket.
Such devices with screen baskets essentially serve to remove disturbing ingredients of fiber suspensions, for instance lightweight, floating contaminants, such as plastic foils and the like, heavy ingredients, such as sand, glass splinters, wood chips, and iron parts of essentially small type, such as staples, wire pieces, etc. This is accomplished by a suitable dimensioning of the screen perforation or slot width of the screen or screen basket so that only the good fibers or also fiber bundles can proceed into an accepts space.
A good sorting efficiency requires that additionally the following conditions be met:
1. The generation of gravity forces and the maintenance of a turbulent movement in the suspension is necessary in order to prevent the formation of fiber flakes, especially at solid substance concentrations of more than 0.8%, and a demixing of the suspension in fibers and water on the screen. In the latter case, a thickening would occur on the screen that would prevent the passage of further accepts through the screen openings.
2. The generation of pressure pulsations on the screen so as to eliminate or prevent cloggings of the screen openings through, for example, fiber flakes and foreign bodies.
Recently, attempts have been made at performing the sorting operation at maximally high substance concentrations. As a result, novel rotors have been developed, of which the arrangement according to the U.S. Pat. No. 4,200,537 is an example. Such rotors have a good sorting efficiency but stress the screen basket quite heavily.
The problem underlying the invention is to provide a rotor which, while having a good sorting efficiency, causes only minimal stresses on the screen basket.